It is well known to control the current flow to electrical loads, for example d-c loads, in accordance with control signals. For example, the field circuit of a shunt d-c dynamo electric machine for public transport use, for example for electric buses or other traction use can be connected directly to the catenary voltage, if the duty cycle of the current flow is controlled by a thyristor. The turn-off circuit for the thyristor can be in accordance with well known circuits of this type. For turn-on, however, it is necessary that the isolation between the thyristor and the thyristor control circuit can meet the highest requirements, due to safety reasons. For example, a leakage current of less than 1 mA is permitted at a test voltage of 9 kV for typical public use or public transit applications.
It has been proposed to control the thyristor by means of firing pulse transducers which have high insulation quality due to the presence of a shield winding. These thyristor firing arrangements, with a transformer, for example, require high manufacturing costs, are heavy, and have a limited transfer characteristic with respect to the speed or rate of rise of the firing pulse as well as of the voltage-time integral.